1. Field of the Invention
In regenerative air preheaters a mass of heat absorbent material commonly comprised of packed element plates forming a heat transfer matrix is positioned in a hot gas passageway to absorb heat from the hot gases passing therethrough. After the plates become heated by the hot gas they are suspended in a cool air passageway where they give up their absorbed heat to the cool air flowing therethrough.
As the hot exhaust gases are directed through the heat exchange apparatus, fly ash and unburned products of combustion carried by the exhaust gas are deposited on the surface of the packed element plates. These deposits continue to be deposited and to build up on the surfaces of the element plates until the flow passageways therethrough are blocked and all fluid flow through the heat exchanger is substantially stopped. Heat is then generated in the element itself until the deposits begin to glow and cause a "hot spot" that if not detected and promptly cooled will rapidly increase in size and temperature until the metal of the heat exchanger will itself ignite and cause a catastrophic fire.
2. Description of Prior Art
Recent developments in the use of infra-red ray detection apparatus to disclose the existence of "hot spots" in heat exchange apparatus in the manner disclosed by U.S. Pat. Nos. 3,730,259 of 1973, and 3,861,458 of 1975, have been successful in fulfilling their stated objective of signalling a potential fire or "hot spot" well in advance of the occurrence of a damaging fire.
In actual use, however, it has been found that mounting a typical infra-red detector including a lens therefor in an air preheater simultaneously subjects the detector and the lens to a constant stream of corrosive gases and particulate matter. Any viewing means exposed to such an atmosphere quickly becomes clouded, it fails to rapidly detect a change of infra-red ray emission, and it results in a loss of viewing efficiency. Therefore, it has been determined that any response of such apparatus to a variation in infra-red rays being emitted by a "hot spot" is directly dependent upon the cleanliness of the lens and the detection device.